Hafnium alloys



United States Patent M US. Cl. 75-134 9 Claims ABSTRACT OF THEDISCLOSURE Hafnium base alloys of improved tensile strength and suitablefor nuclear reactor control rods consist of, by weight, 0.1% in totalamount of one or more of the beta-stabilising elements niobium,chromium, molybdenum, nickel and iron, up to 0.5% silicon, up to 2% intotal amount of at least one of the alpha stabilising elements tin andaluminum with the total alpha and beta stabilisers not exceeding 5%,balance hafnium and impurities.

This invention relates to hafnium alloys.

Hafnium is used for control rods in nuclear reactors because of its highneutron absorption coefficient. Its tensile strength is not great (about25 tons/sq. in.) but it is very ductile and has good corrosionresistance.

We have found that the tensile strength can be improved by certainalloying additions without significant deterioration of corrosionresistance, nuclear properties or ductility.

According to the present invention, a hafnium-base alloy consists, byweight of 0.l-5% in total amount of one or more of the beta-stabilisingelements niobium, chromium, molybdenum, nickel and iron, 0-0.5% siliconand optionally, 02% in total amount of one or both of the alphastabilising elements, tin and aluminum, the total amount of alpha andbeta stabilisers not exceeding 5%, balance hafnium and impurities.

The impurities include zirconium in amounts up to about 4%, thisunusually high impurity level being due to the common mineralogicalsource of hafnium and zirconium. Such a quantity of zirconium does notadversely affect the properties and hafnium having a high zirconiumcontent is cheaper than purer grades.

The effect of the beta stabilising elements is to strengthen hafnium;the alpha stabilising elements, whilst having little effect on strengthwhen added alone, generally improve the properties of the betastabilised alloy.

The addition of small percentages of beta stabilisers produces markedimprovements in tensile strength and proof stress whilst good ductilityis retained. There is a difference in the effectiveness of individualbeta stabilisers in raising the tensile properties and molybdenum isparticularly effective in this respect. Where more than one betastabiliser is present, certain combinations are more effective thanothers, for example, chromium and silicon are more effective thanniobium and molybdenum which, in turn, are better than chromium andmolybdenum.

The effect of alpha stabilisers on tensile properties is a further smallincrease in tensile strength and proof Patented June 2, 1970 stress withsome loss of ductility. Certain groups of alpha and beta stabilisers aremore effective than others as can be seen from the tables appendedhereto and the more complexalloys, in general, have the greateststrength though the ductility and proof stress Vary with the combinationof elements. The best alloys have twice the tensile strength and proofstress of hafnium with half the ductility.

From the results of tests recorded in the tables, preferred ranges ofcomposition are as follows:

O.15-0.5% Ni 0.6-0.8% Cr and 0.81.2% Si 0.6-O.8% Cr and 0.75-2.0% SnCorrosion resistance depends upon the alloying elements added and theamount present. Chromium and nickel have least effect whilst molybdenumhas a marked effect on the surface appearance. Certain of the morecomplex alloys have better corrosion resistance than certain binaryalloy compositions, but the difference between pure hafnium and alloyshaving the least corrosion resistance to high temperature pressurisedsteam is one of small degree.

The corrosion resistance of the alloys of the invention was determinedby exposing cleaned bright samples to steam at a pressure of atmospheresand at a temperature of 400 C. for 72 hours. The corrosion behaviour ofthe alloys was compared by the appearance of the surface of the samplesin accordance with the following classification.

(A) Glossy, bright, thin, adherent film (including interference films).

(B) Glossy, bright, thick, adherent film.

(C) Mottled appearance with dull patches or streaks which are usuallypowdery.

(D) Dull but adherent uniform film.

It is important that the film should be adherent with no tendency topowder or to flake off. In the above classification, class (A) has thebest corrosion resistance.

Because the alloying elements are present in small amounts, the nuclearproperties are not significantl affected.

The tensile properties and corrosion resistance of a number of alloys inaccordance with the invention are shown by way of example in Tables Iand II in comparison with pure hafnium. The tensile tests were carriedout on rod and in the tables the abbreviation 4 /A, used in connectionwith the elongation values, refers to the gauge length of test pieces,that is, a gauge length equal to four times the square root of the areaof the cross-section of the test piece. The corrosion resistance wasdetermined in accordance with the above classification.

In Table III are shown examples of the improvement in the strength ofsheet produced from alloys in accordance with the invention comparedwith commercially pure 3 It will be observed that the alloy B(L),Hf-1.1Sn-O.6Cr is heat-treatable to produce marked increases in tensileTABLE III,,TENSILE PROPERTIES OF STRIP strength and proof stress Whilstthe ductility remains at a 55 2} 02% high level. Analysed spcci- I.S.,U.T.S., tion on Alloy composltion men t.s.i. t.s.i. 1 in. 5 A(R) .055 Orl i ongufl 331; rams..- TABLE I 13(L) 1.1 Sir-0.6 C1 Long 16. 6 30.9 40Hafniuin rich alloys containing 15% of one or more of the 12}? betastabilising elements Cr, Mo, Nb, Ni, Fe 01' 0.5% Si COM YONG/d 0- 3 34 066 and heat treat- Elongation ed 1 hr. 750 0. Nominal Corrosion 0.2%P.S., U.T.S., percent on. and 1 Alloy composition resistance t.s.i.t.s.i. 4 v1; 850 C.

Pure hafnium Long... 9. 4 25.3 35 Unalloyed hafnium 15. 6 25. 3 26Trans... 14. 59 24. S9 41 A 23. 7 32.2 29 A 25. 5 27.1 We claim. i ii;23:3 g8 1. A hafnium-base alloy consisting by We ght of 0.1 A 23.1 34. 61s 5 In total amount of at least one beta-stabilising element 8 ,3 3 3:?33 selected from the group consisting of niobium, chromium, A 17.1 30.6molybdenum, nickel and iron; up to 0.5% silicon, and up 3%: 3 it; 20 to2% in total amount of at least one alpha stabilising ele- D 46 48.5 20ment selected from the group consisting of tin and alu- 8 35;; it? Q?minium, the total amount of alpha and beta stabilisers not 0 25.2 39.513 exceeding 5%; balance hafnium and impurities. i 1%: 38:2 2. Ahafnium-base alloy according to claim 1 contain- .A. 3 25 ing 0.15-1.25%chromium. i 1 3 1 20 3. A hafnium-base alloy according to claim 1contain- A 25 ing 0.1-0.40% molybdenum.

4. A hafnium-base alloy according to claim 1 containing 0.52.0% niobium.5. A hafnium-base alloy according to claim 1 containing 0.15%0.75% iron.

6. A hafnium-base alloy according to claim 1 contain- TABLE II ing0.l50.5% nickel.

Holmium-rich alloys containing .1-57 of one or more of the betastabilising elements: Cr, Mo, Nb, Ni, Fe, 00.5% Si, together with 02% ofone or A hafmum base alloy accordmg to clam 1 contain more of the alphastabilising elements, Sn, Al ing O.60.8% chromium and (18-12% SiliCOIl.Elonga 8. A hafnium-base alloy according to claim 1 contain- N min 1Corro- 0 7 P U T S tion, per ing 0.6-0.8% chromium and 0.752.0% tin.

o a sion recent on AHOY nomposmon Sistance a LS1 WK 9. A hafnium basealloy according to clalm 1 consist ing of 0.75% tin, 0.05% chromium,0.06% iron and T10 231911-0251 6 gig g 40 0.02% nickel, balance hafniumapart from impurities.

c 30.4 43.9 is g 36.1? i t l 5 References Cited ft 5%: UNITED STATESPATENTS n a detilfined 6 2,234,969 3/1941 Hensel et a1. 751340.75Mo-0.8Sn-0.2Si. 0 39.1 51.5 15 45 2,810,640 10/1957 Bolkcom et a1.134

O.06Fe-0.02N1

RICHARD O. DEAN, Primary Examiner

